The invention generally relates to the field of prostheses for the spinal column, which are intended to correct and stabilize scoliosis or to reposition and stabilize vertebrae in the case of trauma or of a tumour.
Such prostheses, known as “spinal osteosynthesis devices” can be fitted either via the posterior route or by the anterior route. The latter route has, in particular, the feature of allowing direct access to the vertebral bodies, along with many other advantages a summary of which can be found in document FR-A-2 697 744.
Such devices, which allow two or more adjacent vertebrae to be fused together, can be classified into two categories:                those which employ one or more parts in the form of rigid plates or the like, the plates being fixed to the various vertebrae in the region to be treated; and        those which employ one or two metal rods fixed along the spinal cord, using appropriate connecting devices.        
In the latter category, the devices with two rods are often preferred, because they afford greater stability compared with devices involving one rod, and reduce the risk of lateral deformation. What happens is that the two rods are connected together transversely at least at their two ends, so as to form a frame with an overall rectangular shape. This connection can be achieved either using rigid bars (see document FR-A-2 658 413) or using connectors each having two orifices, one for each rod, in which orifices these rods are immobilized after having been positioned by axial sliding and rotation. The connectors are themselves each fixed to one vertebra.
Document FR-A-2 697 744 discloses one example of a device with two rods connected by connectors as has just been described.
These connectors have the disadvantage of being of a complex design. Above all, fitting them together is a lengthy and relatively complicated process.
Document U.S. Pat. No. 5,620,443 shows an example of a two-rod device, the rods being connected one to each other through transverse connectors, each of which being inserted on the head of a bone screw. Each connector is pressed to the bone screw by a nut, and this pressing causes a deformation of the connector which tends to clamp the rods so that they are locked in their lodgings provided in the connector. The rod clamping quality obtained by this connector depends on the connector's ability to get deformed by the screwing of the nut in the bone screw head.